The invention relates to a stable hydroformylation process.
It is known that aldehydes can be produced by a continuous process comprising reacting an olefinically unsaturated compound with carbon monoxide and hydrogen in the presence of a metal-organophosphorus ligand complex catalyst. This process is disclosed in, e.g., ‘U.S. Pat. No. 4,148,830; U.S. Pat. No. 4,717,775; and U.S. Pat. No. 4,769,498. Reaction temperature is an important hydroformylation process variable for several reasons.
It is generally recognized that steady and controlled operation of a commercial-scale hydroformylation plant is highly desirable. It is also clear that accurate temperature control is critical to catalyst life. The problem of temperature control in hydroformylation reactions on a commercial scale has long been recognized. In section 1.2.4 in J. Falbe (ed) “New Syntheses with Carbon Monoxide” (Springer-Verlag, N Y 1980) a summary of the problem with a diagram of erratic temperature behavior is shown. A more detailed analysis is given in E. P. Van Elk, P. C. Borman, J. A. M. Kuipers, G. F. Versteeg; Chemical Engineering Science 56 (2001) 1491-1500) where the complications of stability and dynamic behavior are discussed. Rhodium hydroformylation reactions are characterized by complex kinetics, mass flow issues, and their very exothermic (28-35 kcal (118-147 kJ)/mol olefin) nature, all of which make temperature control quite difficult.
U.S. Pat. No. 4,277,627 teaches several routes of catalyst deactivation including intrinsic deactivation. Operating conditions are specified to minimize the loss of activity with phosphine-based catalysts. Temperature is a key variable that controls the rate of catalyst deactivation.
In addition to its effect on catalyst stability, controlling the temperature can have a significant impact on the efficiency of the process. Lower temperatures give lower reactivity and result in lost olefin through the system. Higher temperatures give higher ligand decomposition and heavies formation rates due to inevitable aldol formation, as taught in U.S. Pat. No. 4,148,830. Other temperature-related effects, such as higher hydrogenation (to alkane or alcohol) and variation of the normal-to-branched (“N:I”) product ratio, may also negatively impact plant productivity.
Generally speaking, to control the temperature, one must control the rate of heat generation and/or the rate of heat removal. At steady state, these two are equal. The rate of heat generation generally will be determined by factors such as the desired plant production rate, and the nature of the olefin (ethylene being highly reactive followed by primary then secondary olefins) and catalyst concentration, to name a few. The production rate and olefin used are generally not changed due to the resulting negative impact on plant economics. Therefore, most of the focus has been on heat removal.
The removal of heat from a heat exchanger is traditionally described by the following equation:Heat Removal=A*U*ΔT  (1)where “U” is a heat transfer coefficient dependant on the conditions on both the process and coolant side of the equipment (viscosity, sensible heat, flow rates, presence of bubbles, etc.), “A” is the surface area available for the heat transfer and ΔT is the temperature difference between the product fluid and the coolant.
The surface area of the exchanger is generally a constant. Large internal cooling coils inside a reactor take up valuable reactor space, so it is common practice to use external heat exchangers on reactors needing a substantial amount of heat removal. See WO 2012/008717 A2, U.S. Pat. No. 4,523,036, U.S. Pat. No. 8,389,774 and U.S. Pat. No. 5,367,106. Increasing the size of the heat exchanger to have a very large surface area will generally give better stability but is expensive, increases the plant footprint, and increases maintenance costs.
There are disclosures that aim at controlling the reactor temperature via manipulation of operating conditions. For example, with the highly active phosphite-based Rh catalyst systems disclosed in U.S. Pat. No. 5,744,650, optimizing the temperature difference, ΔT, between the process and coolant side of heat exchangers is critical to steady temperature control. That patent gives a good overview of practical heat exchanger design used to control hydroformylation reactors but focuses on the coolant side of the heat exchanger. Unfortunately, controlling the temperature of the cooling water adds complexity and expense to the plant construction and operation. It also adds considerable process control response delay, in that changes to the cooling water temperature take time, and then the altered cooling water has to re-establish a temperature at the heat exchanger, which then must establish a new ΔT to show an effect at the reactor. The large masses involved in industrial scale hydroformylation processes greatly increase the response time.
Traditionally the other means to effect heat removal is based on changing the coolant mass flow rate in the heat exchanger. Changing the flow on the coolant side has been viewed as the preferred path since the piping and equipment on the coolant side are generally much smaller than the process side, e.g., 6 inch vs. 20 inch pipes, and involve less expensive metals, e.g., carbon steel compared to stainless steel on the process side.
It is also known that reaction kinetics, which are affected by temperature, have a large impact on process stability. U.S. Pat. No. 5,763,679 teaches that deactivation of metal-organophosphorus ligand complex catalysts caused by inhibiting or poisoning phosphorus compounds can be reversed or reduced by conducting the hydroformylation process in a reaction region where the hydroformylation reaction rate is of a negative or inverse order in carbon monoxide. The presence of both positive and negative order kinetics (as well as varying levels of inhibitors) makes controlling these highly active catalysts very challenging using conventional process control strategies.
U.S. Pat. No. 5,362,917 discloses a method of controlling the stability of hydroformylation processes by varying the flow rate of a synthesis feed gas or the flow rate of a vent gas to maintain a predetermined constant carbon monoxide partial pressure in the hydroformylation process. Since the product isomer (N:I) ratio is dependent on the CO partial pressure, attempting to maintain the CO partial pressure may stabilize the N:I ratio but not the reaction rate at the same time, since the other reagents may be changing as well. Additionally, using one reagent out of three to control the reactivity is limited by the amount of inventory of reagent already in the reactor.
Similarly, U.S. Pat. No. 7,446,231 deals with controlling the reaction by manipulating the reactor total pressure. This attempts to deal simultaneously with several gaseous reagents that impact the kinetics. Instead of setting a fixed CO partial pressure, the total pressure is maintained at a constant propylene feed rate based on the observation that the CO and H2 partial pressure will self-control, and the hope that a steadier process will result. As shown in FIG. 1 in U.S. Pat. No. 7,446,231, the optimal operating region is at the peak of the hydroformylation rate versus CO partial pressure plot, where the highest rate and N:I performance is observed. Unfortunately, operating at this peak is inherently unstable since kinetic models do not account for changing reaction orders (including zero order at the peak itself). Therefore, the technique of U.S. Pat. No. 7,446,231 only applies in the negative order region.
Thus, hydroformylation reactors typically operate in an inherently unstable regime and depend on the reactor control system to maintain stable process control. Conventional hydroformylation reactor temperature control systems have adjusted the cooling water inlet temperature, cooling water flow rate, or a combination of these to control the reactor liquid temperature. FIG. 1 depicts a conventional hydroformylation process. Conventional hydroformylation reactor temperature control systems that adjust the cooling water inlet temperature, as measured by temperature sensor (18), the flow rate of cooling water outlet stream (7), or a combination thereof in order to control the reactor liquid temperature. Hydroformylation reactor liquid temperature, as measured by temperature sensor (11), is compared to the set point by controller (9) for temperature control of the process, and is maintained at a desired steady value. Historically, this control scheme has worked reasonably well, primarily because the first generation, commercial hydroformylation catalysts had relatively low reaction rates, e.g., less than 2 gmoles aldehyde/liter reactor volume/hr, which generated relatively low reaction heat per unit time/volume. However, recently commercialized next-generation hydroformylation catalysts have significantly higher reaction rates compared to prior catalysts. The higher reaction rates result in higher heat generation in the hydroformylation reactor per unit time. The conventional reactor temperature control scheme is too slow for effective reactor temperature control of reactions that use the new hydroformylation catalysts.
In view of the shortcomings of the prior art, it would be desirable to have an improved reactor temperature control process for hydroformylation reactors.